Use separate table to store the thread structure
Avoid full table scan when creating the threaded message list
Optimize setting flags for whole threads
Fix getFoldersAndUids() for threads
Rewrite/optimize some SQL statements
Fix copy/move when not in 'single folder mode'
Close thread view when last message has been moved/deleted
Merge branch 'threading_schema_change'
Add ckChangeLog to about dialog
Update changelog
Add button to show changelog to 'About' dialog